1. Field of the Invention
This invention relates generally to apparatus for protecting a facility and its loads from surges and other disturbances in supply voltage and, more particularly, to a surge protection device such as, for example, an alternating current (AC) surge protective device (SPD) for such loads. The invention also relates to devices and methods for manufacturing a thermal fuse spring.
2. Background Information
Various devices and arrangements are known for protecting loads from surges in supply voltage. Generally, two approaches are used: series and parallel protection. In series protection, a high impedance is used in series with the load during a surge to block or limit surge current. In parallel protection, the surge current is diverted with a low impedance shunt. The present invention provides parallel protection.
It is known to use voltage clamping devices and crowbar devices to provide parallel surge protection. The voltage clamping devices clamp the voltage across the load to a specified level. Common types of clamping devices include metal oxide varistors (MOVs), zener diodes and silicon avalanche diodes. If used alone to provide parallel protection, an MOV, for example, must have a clamping voltage that is above the nominal supply voltage in order that its maximum continuous operating voltage (MCOV) is above the nominal supply voltage. Known crowbar devices include gas discharge tubes and thyristors. These devices normally have a high impedance and switch to a low impedance when a surge in the supply voltage exceeds the breakdown voltage of the gas in the case of the gas discharge tube, or is high enough to activate a trigger circuit to turn the thyristor on.
FIG. 1 shows a prior proposal for a surge protection device (SPD) circuit in which thermal disconnection of a failing MOV is used. The SPD 2 includes a conventional overcurrent fuse 4, three MOVs 6,8,10, and two thermal cut-off(TCO) devices 12,14. The SPD 2 is employed in a conventional AC power circuit 16 including a power input 18 having phase 20, neutral 22, and ground 24 connections, and a power output 26 having phase 28, neutral 30, and ground 32 connections. A circuit breaker or fuse is typically employed upstream from the SPD 2, in order to protect against a fatal failure of the SPD. For example, a circuit breaker or switch 34 is electrically connected in series between the phase input connection 20 and the phase output connection 28.
There are several disadvantages of this prior proposal. First, the reaction time is relatively long. The metal cases of the TCOs 12,14 are warmed by heat generated by the MOVs 6,8,10, with heat transfer from MOV to TCO being provided by radiation through the air. Because air and the MOV coatings are thermal isolators, it takes time for the TCOs to disconnect the MOVs during an abnormal condition of the SPD 2. Second, the surge current rating of the TCOs 12,14 is also a limiting factor. One TCO is typically suitable for only one MOV. Third, the major disadvantage is cost, since one TCO costs several times more than the corresponding MOV, which is to be protected. In order to improve the cost ratio, it is known to employ a combination of a plural MOVs connected in parallel in combination with a single TCO. However, in that case, the TCO does not have a surge current capability equivalent to that of the parallel connected MOVs.
FIG. 2 shows a SPD 36 including a plurality of MOVs 38, which are electrically interconnected in parallel. Each of the MOVs 38 is electrically connected in series with a corresponding one of the fuses 40 in order to provide individual overcurrent protection. Each of the fuses 40 is intended to open circuit when the corresponding MOV 38 fails and electrical current flowing through that fuse exceeds the current rating thereof. However, that current rating might not be suitable to prevent fire on the corresponding MOV 38 in the event that the surge rating of the corresponding fuse 40 exceeds the surge current rating of that MOV. For example, in the event of a catastrophic failure of one or more of the MOVs 38, the SPD 36, and perhaps the protected load (not shown), might be subject to substantial damage from fire and/or explosion. Degradation of MOVs might occur due to various reasons, which might lead to such a catastrophic failure of the SPD. Although this rarely happens, the end user must be protected.
A relatively high surge-rated SPD usually has plural MOVs connected in parallel in order to share surge currents. A failure of MOVs is extremely rare, but it is catastrophic when it occurs. The reasons for such failures are most often due to abnormal overvoltage (e.g., up to about 200% of nominal voltage) or surges.
There are several known proposals for fusing of MOVs in SPDs. For example, it is known to employ a fuse trace (FT) copper conductor on a printed circuit board (PCB) in series with a single MOV, or overcurrent protection per phase (e.g., a standard RK5 fuse, fuse resistor, block fuse, TCO, thermal fuse, surge fuse). However, such fuse traces are functional only up to certain levels of surge current. For example, known fuse traces cannot handle relatively higher surges (e.g., higher than about 6.5 kA) and still remain suitable for disconnecting a failing MOV at a fault condition (e.g., a fault current of less than about 10 A). This presents two opposite requirements for fuse trace design. First, in order to handle relatively higher surge currents, a fuse trace must have a cross-sectional area which is as large as possible. Second, in order to disconnect a failing MOV in a relatively short time, a fuse trace must have as small of a cross-sectional area as possible. Overcurrent protection, alone, is insufficient to meet these two requirements.
Recently, MOV surge ratings are increasing to relatively higher levels. For example, several years ago, 20 mm MOVs were rated up to about 6.5 kA (with a 8xc3x9720 xcexcs standard waveform). Today, 20 mm MOVs are rated up to about 18 kA surge current.
U.S. Pat. No. 4,862,134 discloses an electrical fuse including a fusible conductor welded or bonded onto separated contact surfaces.
U.S. Pat. No. 6,067,216 discloses a circuit arrangement for protecting an electrical component from an undesirable electrical potential. The circuit arrangement includes a protective element, such as a diode, varistor or thyristor, in parallel with the electrical component. When a protective action occurs in response to an inadmissible voltage surge, the protective element, such as a surface mounted diode, experiences breakdown and becomes shorted which causes the fuse to open shortly thereafter. Also, the solder mounting the diode becomes liquefied as a result of heating of the diode. A divided pair of soldering pads is employed for one of the connections to the diode, which results in a more rapid rise of temperature than on an undivided soldering pad for the other connection to the diode.
U.S. Pat. Nos. 5,600,295 and 5,896,080 disclose thermal fuses for circuit substrates and printed circuit boards.
Generally, fusing in SPDs can be internal or external (e.g. inside the SPD enclosure, or circuit breakers and fuses outside the SPD enclosure).
There is a need for improvement of the fusing function in surge protection devices.
A surge protection device includes plural voltage clamping means, such as MOVs, accompanied by a thermal fuse spring and a fuse trace. Preferably, the three components are coordinated in terms of MOV (pre-fault) leakage current, MOV fault current, and MOV surge current rating for safe disconnection of the MOVs under certain failure conditions, such as overvoltage or surge.
As one aspect of the invention, a surge protection device for a load supplied with at least one voltage from a power source comprises: at least two terminals adapted to receive the voltage; and a plurality of surge protection circuits, each of the surge protection circuits comprising: a thermal fuse spring, at least one means for clamping a voltage, and at least one fuse trace, each one of the at least one fuse trace corresponding to one of the at least one means for clamping a voltage, with the thermal fuse spring, one of the at least one fuse trace and a corresponding one of the at least one means for clamping a voltage being electrically interconnected in series between the terminals, in order to form at least one series electrical connection between the terminals, with the thermal fuse spring being adapted to disconnect the at least one series electrical connection between the terminals under first fault conditions of the at least one means for clamping a voltage, the first fault conditions including a first current through one of the at least one means for clamping a voltage for a first duration, with the thermal fuse spring and the at least one fuse trace being adapted to cooperatively disconnect a corresponding one of the at least one series electrical connection between the terminals under second fault conditions of the at least one means for clamping a voltage, the second fault conditions including a second current through one of the at least one means for clamping a voltage for a second duration, with the second current being greater than the first current, and with the first duration being greater than the second duration.
The at least one means for clamping a voltage may be a metal oxide varistor having a catastrophic failure state under overvoltage conditions, and the at least one fuse trace may be a fuse trace. Each of the thermal fuse spring and the fuse trace has a time versus current characteristic which is coordinated with the catastrophic failure state of the metal oxide varistor in order to disconnect the metal oxide varistor from the voltage under the overvoltage conditions. At least one of the thermal fuse spring and the fuse trace disconnects the metal oxide varistor from the voltage in order to prevent the catastrophic failure state under the overvoltage conditions.
The surge protection circuits may include a printed circuit board. The fuse trace may include a first conductive trace on the printed circuit board, a conductive through hole in the printed circuit board, and a second conductive trace on the printed circuit board, with the first conductive trace being electrically connected to the input of the fuse trace and to the conductive through hole, and with the second conductive trace being electrically connected to the conductive through hole and to the output of the fuse trace.
Preferably, the conductive through hole of the fuse trace is filled with solder, in order to shorten a disconnection time of the fuse trace during an overcurrent condition. The solder may have a melting temperature of between about 175 degrees C. and about 250 degrees C.
The surge protection circuits may include a printed circuit board having a first conductor, which is electrically connected to one of the terminals, and a second conductor, which is electrically interconnected with the input of the fuse trace. The first spring member of the thermal fuse spring may be soldered to the first conductor of the printed circuit board and the second spring member of the thermal fuse spring may be stretched away from the first spring member and soldered to the second conductor of the printed circuit board.
Preferably, the first and second spring members of the thermal fuse spring are each soldered to the printed circuit board with a solder having a melting temperature of about 80 degrees C. to about 120 degrees C.
As another aspect of the invention, a method of manufacturing a thermal fuse spring comprises: cutting a flexible conductive material to form a base and a plurality of fingers; bending the base and the fingers to form a plurality of thermal fuse spring elements having a common base and a plurality of independent leg elements; stretching each of the leg elements away from the common base; positioning the common base and the stretched leg elements on a printed circuit board; and soldering the common base and the stretched leg elements to the printed circuit board.
As a further aspect of the invention, a stretcher device is for use in mounting at least one thermal fuse spring on a printed circuit board. The thermal fuse spring includes a spring member having a pair of legs each of which has a hooked foot member. The stretcher device comprises: a base including first and second ends, first and second sides, and an elongated opening in the base; a first alignment member affixed to the first end of the base; a second alignment member affixed to the second end of the base; a first elongated arcuate member affixed to the first side of the base; and a second elongated arcuate member affixed to the second side of the base, with the opening of the base adapted to receive the spring member of the thermal fuse spring, and with the first and second elongated arcuate members adapted to hold the hooked foot members of the thermal fuse spring with the legs thereof being stretched thereapart for engagement with the printed circuit board.
As another aspect of the invention, a surge protection device for a load supplied with a voltage from a power source comprises: a plurality of terminals adapted to receive the voltage; a printed circuit board; and a plurality of surge protection circuits, each of the surge protection circuits comprising: a thermal fuse spring on the printed circuit board, means for clamping a voltage, a fuse trace electrically interconnected in series with the thermal fuse spring and the means for clamping a voltage between the terminals, in order to form a series electrical connection between the terminals, the fuse trace including at least one conductive trace on the printed circuit board and also including a conductive through hole in the printed circuit board and electrically connected to the at least one conductive trace, and with the thermal fuse spring being adapted to disconnect the series electrical connection between the terminals under first fault conditions of the means for clamping a voltage, and with the thermal fuse spring and the fuse trace being adapted to cooperatively disconnect the series electrical connection between the terminals under second fault conditions of the means for clamping a voltage.